text: Most FGD systems employ two stages: one for fly ash removal and the other for  removal. Attempts have been made to remove both the fly ash and  in one scrubbing vessel. However, these systems experienced severe maintenance problems and low removal efficiency. In wet scrubbing systems, the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a baghouse, and then into the -absorber. However, in dry injection or spray drying operations, the  is first reacted with the lime, and then the flue gas passes through a particulate control device.
  Another important design consideration associated with wet FGD systems is that the flue gas exiting the absorber is saturated with water and still contains some . These gases are highly corrosive to any downstream equipment such as fans, ducts, and stacks. Two methods that may minimize corrosion are: (1) reheating the gases to above their dew point, or (2) using materials of construction and designs that allow equipment to withstand the corrosive conditions. Both alternatives are expensive. Engineers determine which method to use on a site-by-site basis.